The invention relates to an apparatus for measuring the position of weft threads in a moving fabric or textile web, whereby to ascertain deviations of the fabric web from a standard, said deviations being indicated by an angular measurement and by a length measurement. German Pat. 1,635,266 discloses an apparatus for measuring the position of the weft threads in a travelling textile or fabric web by means of one or several sensor heads which ascertain deviations of the fabric or textile web by an angular measurement and by a length measurement. In this prior art device each sensor head scans only a fraction of the total length of the web threads or of the mesh rows or the like, whereby the angle is ascertained which this fractional length of the entire weft thread length takes up relative to a normal position. Such scanning of short weft thread lengths is sufficient for recognizing the entire characteristic of uniform deviations. Thus, it is possible with the prior art apparatus to ascertain with a single scanning or sensor head uniform, steady, slanting deviations. Uniform, bent bow-shaped deviations require at least two scanning heads arranged symmetrically relative to the center of the fabric web. In both instances the deviation is determined with regard to its direction and with regard to its value without any difficulty.
However, where the deviations are not uniform, but rather non-symmetric or wavy, as happens frequently in practice, and where the deviations are composed of bow-shaped components as well as slanted components, the prior art apparatus is not capable of providing an information regarding the actual weft thread course unless a combination of a plurality of sensing heads uniformly distributed over the width of the fabric web is employed for obtaining such information. The certainty or rather the accuracy of the result increases with the number of sensing heads.
Certain types of textiles such as coarse curtain textiles, canvas, textile wall coverings and the like are structured so loosely that especially any treatment of the fabrics in the non-spread out condition may easily cause a waviness of the weft threads in the weft direction. Such waviness may, for example, occur when the fabric is subjected to bleaching while still in a hang or when the fabric is exposed to humidity. Such waviness may be so large and have such a short wave length that a reliable scanning with a limited number of scanning heads becomes practically impossible.